Industrial Evaporators

Industrial Evaporators

About Our Industrial Evaporators

An evaporator is used to evaporate a volatile solvent, usually water, from a solution. Its purpose is to concentrate non-volatile solutes such as organic compounds, inorganic salts, acids, or bases.

Types of Solutes

Typical solutes include phosphoric acid, caustic soda, sodium chloride, sodium sulfate, gelatin, syrups, and urea. In many applications, evaporation results in the precipitation of solutes in the form of crystals, which are usually separated from the solution with cyclones, settlers, wash columns, elutriation legs, filters, or centrifuges.

Precipitates Examples

Examples of precipitates are sodium chloride, sodium sulfate, sodium carbonate and calcium sulfate. The desired product can be the concentrated solution, the precipitated solids, or both. In some applications the evaporator is used primarily to recover a solvent, such as potable water from saline water. No matter the case, the relatively pure condensed water vapour from many evaporators is recovered for boiler feed makeup, salt washing, salt dissolving, pump seals, instrument purges, equipment washing, line washing and many other uses.

Calandria Evaporator

The Swenson calandria evaporator is applied less often today than it was years ago, although there are still a number of companies that prefer this evaporator for various applications. For most applications, however, the lower equipment cost for other designs has prompted the replacement of calandria evaporators with LTV falling-film, LTV rising-film, and forced circulation evaporators.

The calandria evaporator has a heat exchanger (with tubes usually less than six feet long) integral with the vapour body. The level is maintained in the upper portion of the tubes or above the top tubesheet and the circulation pattern is up through the tubes and down through a central pipe called a “downcomer”. Circulation is created by the difference in specific gravity between the body liquor and the headed liquor and vapour generated inside the tubes, plus a vapour lift effect.

The Swenson calandria evaporator can be used for salting-type applications; however, an agitator located inside or beneath the downcomer is recommended to suspend the salt crystals in the lower portion of the body. Although the agitator creates some flow through the tubes, most of the flow is still created by “thermo-syphon”. The calandria is also used for batch-type concentration of liquors.

Forced Circulation

As the name implies, liquor in a forced circulation evaporator is pumped through the tubes to minimize tube scaling or salting when precipitates are formed during evaporation. A Swenson forced circulation evaporator (with a submerged inlet) complete with a single-pass vertical heat exchanger, elutriating leg, cyclone, and top-mounted barometric condenser is shown in the photo.

Slurry is pumped from the bottom cone of the vapour body through the tubes of the vertical heat exchanger, where heat is added, and back into the vapour body where evaporation occurs. Sufficient slurry height (submergence) is maintained above the tangential inlet on the vapour body and above the top tubesheet of the heat exchanger to suppress mass boiling in the inlet and prevent surface (local) boiling on the tube surface. This is necessary to preclude salt precipitation on the tangential inlet and tubes.

A high circulation rate is provided for adequate tube velocity to achieve good heat transfer. Therefore, lower slurry temperature rises are assured which minimize supersaturation of the solution.

A sufficient quantity of salt crystals is suspended in the circulating system to provide seed crystals in the boiling zone for salt growth. Adherence to these basic principles of crystallization results in coarse crystals and minimal wall and tube salting, so less equipment washing is required. This conserves energy because less steam is required to boil wash water and this increases on-stream time for the evaporator.The circulating pump is usually of the axial-flow, single-elbow design, well-suited for the high flow rates and low pressure drops in Swenson designed circulating systems. These heavy duty pumps operate at low speeds, which reduce maintenance and minimize mechanical attrition of the salt crystals.

The tangential inlet provides excellent mixing of slurry in the vapour body because of the circular motion it creates. Secondary vertical currents are also generated, mixing body slurry with the hotter slurry entering the vapour body to reduce the degrees of flash. This agitation minimizes salt buildup on the bottom cone of the vapour body. A swirl breaker is provided in the circulating slurry outlet.

The vapour body is conservatively designed both in diameter and height. It is important to have adequate free space above the liquor level to allow the liquor droplets entrained in the vapour leaving the boiling surface to reach equilibrium and return by gravity to the circulating slurry. Large diameters result in low upwards vapour velocities which minimize entrainment and provide adequate retention time for salt growth.

A mesh type entrainment separator may be installed in the upper portion of the vapour body to reduce solids carryover to normally less than 50 parts per million parts of vapor. Other types of entrainment separators are also available.

The elutriating leg, attached to the bottom cone of the vapour body, is a convenient device for thickening the slurry it receives from the vapour body and for salt crystal washing and classification. Slurry enters the top of the leg through a unique slurry-inlet device, which improves washing efficiency by reducing agitation in the leg. Salt crystals are fluidized and washed in the leg with a portion of the feed liquor which enters the bottom cone and is distributed with a perforated, dished head. Smaller crystals are washed into the vapour body for additional growth and the larger crystals are discharged from a connection near the bottom of the leg.

A Swenson-designed, low-pressure-drop liquid cyclone is sometimes used to clarify liquor discharged from the evaporator. The driving force is the pressure drop across the circulating pump. Thickened slurry is returned through a wide-open cyclone underflow connection to the circulating piping before the pump suction.

Another Swenson innovation is the direct-contact condenser mounted on the vapour body. A short piece of vertical pipe connects the vapour body with the condenser to minimize piping and pressure drop. This design also eliminates structural steel for support of a separate condenser.

LTV Falling-Film Evaporator

A Swenson single-effect, LTV falling-film evaporator with a separate vapour body and heat exchanger is shown. Liquor is fed into the top liquor chamber of the heat exchanger where it is disturbed to each tube. Swenson provides several different distribution devices for falling-film evaporators; a distribution plate is shown. The liquor accelerates in velocity as it descends inside the tubes because of the gravity and drag of the vapour generated by boiling. Liquid is separated from the vapour in the bottom liquor chamber of the heat exchanger and with a skirt-type baffle in the vapour body. A supplemental entrainment separator can be installed in the upper portion of the vapour body to reduce liquid entrained with the vapour to a minimum. The Swenson direct-contact condenser, as shown, is used to condense the vapour with water. Concentrated liquor is discharged from the bottom liquor chamber and cone bottom of the vapour body.

Evaporation occurs inside the tubes of the falling-film evaporator. The unit can be used to concentrate the same non-scaling liquids concentrated in rising-film evaporators, and it is suitable for concentrating more viscous liquors. Tubes are normally ¾” to 2” in diameter and from 10 to 30 feet long. The falling-film evaporator is particularly useful in applications where the driving force in temperature difference between the heat-transfer medium and the liquid is less than 15°F. The retention time for the liquor in this evaporator is less than that for a rising-film, and this fact paired with lower temperature differences makes the falling-film evaporator ideal for concentrating the most heat-sensitive materials.

LTV Rising-Film Evaporator

In a Swenson single-effect, LTV rising-film evaporator, evaporation occurs inside the evaporation tubes, so it is used primarily to concentrate non-salting liquors. To provide for good heat transfer, the temperature difference between the heating medium and the liquor should be greater than 15°F. Tubes are normally ¾” to 2″ in diameter and from 10 to 30 feet long. Liquor is fed into the bottom liquor chamber and enters the tubes. There it is heated with condensing steam or any other suitable heat-transfer fluid. If the vapour pressure of the feed equals or exceeds the system pressure at the bottom tubesheet, vaporization will occur immediately. For colder feed, the lower portion of the tubes is used to preheat the liquor to its boiling point. Vaporization then begins at the height within the tubes where the vapour pressure of the feed liquor equals the system pressure.

As the liquor climbs up the inside of the tubes, additional vapour is generated and the velocity of the liquid-vapour mixture increases to a maximum at the tube exit. The outlet mixture impinges upon a deflector, mounted above the top tubesheet of the heat exchanger, where gross, initial separation of the liquid from the vapour occurs.

Additional entrained liquor is separated from the vapour by gravity as the vapour rises in the vapour body. A mesh or centrifugal entrainment separator can be installed near the top of the vapour body to remove most of the remaining traces of liquid from the vapor. The exit vapour is routed to either the next effect of a multiple-effect evaporator system, to a compressor or to a condenser. A Swenson vertical-tube surface condenser is shown in the Figure 1. The concentrated liquor is discharged from a connection near the bottom of the vapour body.

Mechanical Vapour Recompression

Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapour from an evaporator is compressed (with a positive-displacement, centrifugal or axial-flow compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger. Various combinations are possible, including single-effect recompression, multiple-effect recompression, multiple-stage recompression, and single-effect recompression combined with a multiple-effect evaporator.

A simplified flowsheet of a single-effect recompression evaporator illustrates why mechanical recompression is energy efficient. All of the pertinent pressures and temperatures are shown in the table below for this example.

Based upon a 75% isentropic (adiabatic, reversible) compressor efficiency and a combined electric drive motor and gear reducer efficiency of 92%, the energy required to compress a single pound of vapour from 14.1 to 22.8 psia is only 49.3 BTU. To produce the equivalent steam from one pound of 234°F evaporator condensate requires 999 BTU. Therefore, the energy savings for a recompression evaporator are highly competitive with those of multiple-effect evaporators and depend upon the compression pressure ratio required and the relative cost of electric power and steam.

The compression ratio required is comprised of three components which are:

The boiling-point rise, i.e., the temperature of the boiling liquor minus the temperature of boiling water at the same pressure

Shown above, is a simplified flowsheet for a single-effect recompression soda ash evaporator which has replaced the traditional triple-effect evaporators used for this application. The vapour body shown has the alternate Swenson vertical-inlet baffle design, which has proven to be effective in minimizing short-circuiting. Vapour from the body is compressed with a single-stage centrifugal compressor and condensed in the vertical heat exchanger. Condensate is sprayed into the vapour discharged from the compressor to reduce super-heat. Some make-up steam is required to supplement the mechanical energy from the compressor. For some applications, make-up steam is not required.

Most submerged-inlet evaporators short circuit. That is, some of the heated liquor which enters the vapour body short circuits to the outlet instead of rising to the boiling surface. The boiling temperature of the liquor is increased above the equilibrium value (denoted as degrees of short-circuiting), which decreases the Delta-T available for heat transfer. It is particularly important to minimize short-circuiting in recompression evaporators because short-circuiting increases the compression ratio required, thereby increasing power consumption.

The following table compares the energy required for this system versus that required for a triple-effect evaporator:

UTILITIES REQUIRED PER ONE TON OF SODIUM CARBONATE MONOHYDRATE

Triple Effect

Recompression Evaporator

Power, kwh

Compressor

0

46.7

Circ. Pumps

9.07

12.9

Total

9.07

59.6

Steam, lb

1,580

636

Condenser H2O, gpm

75.3

0

Steam & Power cost

$9.93

$6.80

(Based upon $6 per 1,000 lb steam & $0.05 per kwh power)

Mechanical recompression is not limited to single-effect evaporation. It is sometimes economical to compress vapour from the last effect of a double- or triple-effect evaporator so that the vapour can be condensed in the first-effect heat exchanger.

The vapour recompression evaporator (above) uses high pressure steam (880 psia, 900°F) fed through two turbines in parallel; one turbine drives a generator to produce electricity for the plant and the other turbine drives a multi-stage, axial-flow compressor for the single-effect recompression evaporator. The 44 psia steam discharged from both turbines is condensed in the first effect of the quadruple-effect salt evaporator and a small quantity is condensed as make-up steam in the recompression evaporator heat exchangers.

Thermal Vapour Recompression

To reduce energy consumption, a portion of vapour from an evaporator is compressed with high pressure steam in a thermocompressor so it can be condensed in the evaporator heat exchanger. The resultant pressure is intermediate to that of the motive steam and the water vapour. The remaining vapour is condensed in the next heat exchanger or condenser.